Had an Avon rep tell the ex (unofficially) that we could put a 15hp two stroke on our 8hp rated dink so we could plane heavier loads. He only had two caveats: No high speed turns and no jumping off wakes, waves, etc. It worked out well, although I discovered one more caveat: If you lend it to somebody Warn Them! Thing was a rocketship.

IMO, overpowering a RIB is one thing, but an air-floor is quite another. I suspect that one will find the air-floor isn't stiff enough to handle the thrust of the bigger motor without bending and spoiling the hull shape. I know that I have seen this with a slatted "roll-up" type dink that only had the recommended max power fitted.

At the very least, you will need to maintain the air pressure in floor and tubes right at the max to optimize stiffness.

Cheers,

Jim

__________________
Jim and Ann
s/v Insatiable II, now back in Port Cygnet for a few days.

I think with that much weight and power it'll just dig a hole and unless you have someone sitting right up the front and you half way it will struggle to plane.

I've got a 10ft all rubber Zodiac copy (alloy floor but fabric hull), 8 hp. Planes well but needs the weight well forward to get on the plane. Once on the plane, quite a fast machine. It weighs 100 pounds all up.

I put a 5hp on our dinghy (9' alloy) which would be rated for 6 hp max.

A little while ago, on a little trip across the bay, while fiddling with the tiller extension I inadvertently put it hard over.

That little boat started spinning round in circles. It took an eternity to get my balance back and to straighten the tiller.

My senior crew is still with me, but she's never been keen on having the outboard on the dinghy ever since.

If I'd had a 15, or even a 9.8 I might not be with us now, or at the very least I'd be cruising alone.

I treated it lightly, saying to accellerate slowly, but I actually had a friend in college who lost her fiance to this. He was teaching her how to use the dinghy, and she goosed it too hard, too fast, and he literally flew out of the boat, spraining his back. He couldn't swim injured and she didn't have enough boat handling skills, and she just couldn't figure out how to get back to him in time.

I don't think he had a life jacket on ... this was over 40 years ago, after all -- but it can be very dangerous. My motor is 6hp, and my dingy is rated for 5, and ... I don't take it up to full throttle. They knew what they were doing when they rated my dinghy.

Alex, it came with the boat and is fairly new, so i either sell it or use it till it falls to bits. I know an 8hp would be ideal for it, would plain with two and not overpower, but i intend to get a small rib later and would an 8hp be enough for a small 10ft rib? ie to plane with 2?

Or would it be better to just get the 15hp i have been offered and just be careful....

The main factors in the maximum recommended power calculation are:
- The beam at the transom (an indicator of the boat's ability to support the weight of an engine, and to remain stable when the helm is hard-over under power), and
- One of several quantities that are calculated from the total volume of the hull (an indicator of the boat's weight, and therefore how it will accelerate).

Too much engine relative to the transom beam means the boat is likely to either capsize in a turn at speed, or be swamped by its stern wave in a sudden stop.

Too much power relative to the total weight or hull volume means the boat is likely to catapult passengers overboard under hard acceleration or in a sudden stop.

The strength of the transom is not a factor in the maximum power calculation. The power rating is calculated first, purely from the hull geometry, and the transom structure is then designed according to the maximum thrust and engine weight for that power rating.

This is not to say over-powering is not possible: the ratings do err on the conservative side. But over-powering usually results in a twitchy, squirrely boat that needs a careful hand on the tiller and throttle, and can't be loaned out to unsuspecting guests without a lot of on-the-water training.

For our OP's Wetline 265, using a 15 hp, or almost double the recommended 8 hp, is a Bad Idea. If you need that much power and speed, you need to trade it in for a larger dinghy. Period.

edit- Steven, in response to post #24, 8 to 10 hp will plane a 10 to 14 foot shallow-V rigid dinghy with two adults, a cooler and a few bags of groceries. I've planed a 14-footer with 10 hp and a pair of 24-gallon propanetanks (me plus the tanks = close to 500 lb).

Alex, it came with the boat and is fairly new, so i either sell it or use it till it falls to bits. I know an 8hp would be ideal for it, would plain with two and not overpower, but i intend to get a small rib later and would an 8hp be enough for a small 10ft rib? ie to plane with 2?

Or would it be better to just get the 15hp i have been offered and just be careful....

decision made!!! get the 15hp if its a bargain ,but reccomend keeping/using the smaller engine on it,untill you get the bigger rib.,then keep the other as a spare.

even on my 35 ft boat i had two dinghys and two engines.

we used to have a 2 hp for our bigger rib when we couldnt be bothered with the hassell of putting the 25hp on

a 10 ft will still plane 2 up with a 8 hp though.

also if it still proves too big you can allways swap it for a smaller engine with someone like me in the anchorage.................

Alex. its a mercury 15hp 2 stroke 2007 , spotless and not even enough hours to be run in, for 600 quid, a bit of a bargain i thinks, weighs 34kg, just light enough for me to manhandle without a hernia, most 8hp four strokes are heavier!